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Chapter 3

Psychology 1000 Chapter 3.docx

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Psychology 1000
Terry Biggs

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Psychology 1000 Chapter 3: Biological Foundations of Human Behavior September 18, 20, 25, 27/2012 Prof: Dr. T Biggs Outline  The organism as machine - Descartes & the Reflex concept  Basic Nervous Functions - Reception, integration & reaction  Nerve Cell and Nerve Impulse - The Neuron  Interaction among nerve cells - The reflex – mechanics - Inference of the synapse - Synaptic mechanism  Major structures of the Nervous System - Evolution of central control, peripheral & central (CNS) systems  Cerebral Cortex and Brain Laterality THE ORGANISM AS MACHINE The Reflex Concept  Descartes was first to seriously propose that humans be studied as one would study any machine  His views are still with us today as we employ metaphorical comparisons between the human mind and computers  To Descartes ALL action was as a response to some external event  Chain of events:  Stimulation of a sense – relay to brain – interpretation via the soul – relayed to muscle for action  Thus Energy is taken in and REFLECTED back out  Note: use of the soul provides a means to account for inconsistency of responses Basic Nervous Functions  We agree with Descartes that the bulk of human behavior is reflexive  This necessitates a Tripartite system 1. Reception – via the senses 2. Reaction – via the muscles & glands 3. Integration – mediator between Reception & Reaction via Conduction The Tripartite System  The external event is called the Stimulus as it stimulates the Receptors (Transducing nerves)  The receptors convert the energy and channel it to bundles of nerves call AFFERENT nerves (sensory)  Afferent nerves may lead to the spinal cord and travel to the brain to INTERNEURONS that connect to EFFERENT nerves (motor) which carry signals to Muscles & Glands, or they may in some minority of instances, connect directly to Efferent nerves Basic Building Blocks The NEURON  Three main parts 1. Dendrites  Receive messages from other neurons 2. Cell Body  Contains the genetic information determining cell Motor Neuron function 3. Axons Spinal chord  Conducts electrical impulses GLIAL CELLS  Surround neurons and hold them in place  Manufacture nutrient chemicals neurons need Pyramidal cell  Absorb toxins and waste materials cortex Nerve Conduction  Neurons have a resting potential of -70 millivolts, creating a state of polarization - Neurons are surrounded by a salty liquid environment which has a high concentration of sodium ions (Na ) + - The inside of the neuron has some positively charged potassium ions (K+) and many other negatively charged ions - The combination creates the resting potential Nerve Conduction: THE ACTION POTENTIAL 1. Dendrites are stimulated by axons of other neurons 2. Na+ ions flow into the neuron by action of ion channels, making the membrane voltage more positive (depolarization) 3. If the partial depolarization reaches -.65 millivolts (action potential threshold), the neuron fires according to the all-or-none law Nerve Conducti on: STIMULU S INTENSITY  Given the action potential is all or none, how do we distinguish between different sensations?  E.g., the buzz of a mosquito vs. the roar of a jet engine  This is the realm of stimulus intensity  Two mechanism allow for the determination of Stimulus intensity 1. The number of neurons firing – Neurons differ in their thresholds. Thus the mosquito will cause X number of neurons to fire while the Jet will cause X+ neurons to fire 2. Frequency of Impulse - The duration of a signal will also affect the nerves firing as prolonged signal will result in a 2 , 3 , 4 … Action potentials - The stronger the stimulus (Intensity and duration) the more OFTEN the nerve will fire - The maximum rate is 1000 impulses per second Nerve Conduction: THE MYELIN SHEATH  Insulation layer covers axons in the brain and spinal cord  Allows for high-speed conduction  Multiple sclerosis occurs when immune system attacks the sheath Ne rve Conduction: EFFECTS OF NERUOTRANSMITTERS  Excitatory neurotransmitters depolarize the postsynaptic cell membrane by stimulating the inflow of sodium ions  Other neurotransmitters hyperpolarize the membrane by stimulating ion channels to allow K+ ions to flow out, thus inhibiting neuron firing Nerve Conduction: THE SYNAPSE  Inference of the synapse  Prior to the work of Sherrington transmission of nerve signals was not well understood  Sherrington’s aim was to study the Simple Reflex  He employed dogs whose spinal chords were severed just below the neck  This ensured that any responses to stimulation were Afferent to Efferent only and didn’t involve the brain and interneurons  He found that 1 sub (below) threshold stimulation didn’t cause any response but several sub threshold stimulations in a row at ½ second intervals did cause a response  This is important as it revealed the nervous system was capable of TEMPORAL SUMMATION and the half second delay was too great to allow for this in 1 neuron  Thus temporal summation must occur OUTSIDE the axon  Sherrington proposed that this occurred via chemical production and buildup between an axon and a dendrite  Sherrington also reported that the same summation effect could be found if several Temporal Summation sub threshold stimulations were applied simultaneously to different spots within a proscribed region  This is important as it demonstrates SPATIAL SUMMATION of nerve signals  This suggests that several nerves axons may converge on the dendrites of one other nerve cell  Since each stimulation alone produced no response he argued this supported the chemical production and pooling outside the axon – The area is the Synapse and the gap is the SYNAPTIC CLEFT Spatial Summation  The chemicals that are produced are called NEUROTRANSMITTERS  They are produced at the terminal endings of the axon and released into the synaptic cleft  They are taken up
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